Markus Vester

Duplexer including a field-effect transistor for use in an ultrasound imaging system

A duplexer for an ultrasound imaging system is capable of selectively coupling a transducer element to either a transmitter or a receiver, depending on whether the imaging system is in the transmit mode or receive mode. The duplexer contains a field-effect transistor whose source and drain are connected between the transducer element and the receiver. The gate of the field effect transistor may either be driven with a control voltage or short-circuited to the source. In this manner, one obtains an improved duplexer that operates with virtually no power loss, without drive circuits, without leakage currents, and with low noise.

Degenerate mode band-pass birdcage coil for accelerated parallel excitation.

An eight‐rung, 3T degenerate birdcage coil (DBC) was constructed and evaluated for accelerated parallel excitation of the head with eight independent excitation channels. Two mode configurations were tested. In the first, each of the eight loops formed by the birdcage was individually excited, producing an excitation pattern similar to a loop coil array. In the second configuration a Butler matrix transformed this “loop coil” basis set into a basis set representing the orthogonal modes of the birdcage coil. In this case the rung currents vary sinusoidally around the coil and only four of the eight modes have significant excitation capability (the other four produce anticircularly polarized (ACP) fields). The lowest useful mode produces the familiar uniform B1 field pattern, and the higher‐order modes produce center magnitude nulls and azimuthal phase variations. The measured magnitude and phase excitation profiles of the individual modes were used to generate one‐, four‐, six‐, and eightfold‐accelerated spatially tailored RF excitations with 2D and 3D k‐space excitation trajectories. Transmit accelerations of up to six‐fold were possible with acceptable levels of spatial artifact. The orthogonal basis set provided by the Butler matrix was found to be advantageous when an orthogonal subset of these modes was used to mitigate B1 transmit inhomogeneities using parallel excitation. Magn Reson Med 57:1148–1158, 2007.

METHOD FOR MATCHING ANTENNAS IN A NUCLEAR MAGNETIC RESONANCE IMAGING APPARATUS

In a method for matching a transmission/reception antenna in a nuclear magnetic resonance imaging apparatus to the impedance of a high-frequency system which feeds the antenna, a directional coupler is used for acquiring respective signals corresponding to a forward voltage wave supplied to the antenna, and the associated return voltage wave. The amplitudes of the forward and return waves are calculated in a controller for all matching conditions of a transformation network, the transformation network being interconnected between the high frequency system and the antenna. When a tomogram is to be produced, the transformation network is set so that the antenna is matched as best as is possible to the high-frequency system. The same method can be used for matching an antenna which is only used for reception when producing tomograms.

HTS flux pump for cryogen-free HTS magnets

A reliable method for persistent-current mode operation of HTS magnets is still not available. As an alternative solution to external high-current power supplies we have developed a cryogen-free operating transformer-rectifier type HTS flux pump. Using this device only small ac currents have to be supplied into the cryogenic system. Both thermally triggered HTS thin film switches as well as cryogenic MOSFET switches have been used to operate a conduction-cooled 0.5 T HTS magnet coil. This paper presents results of experimental test and modeling of the flux pump operation.

On-coil multiple channel transmit system based on class-D amplification and pre-amplification with current amplitude feedback

A complete high‐efficiency transmit amplifier unit designed to be implemented in on‐coil transmit arrays is presented. High power capability, low power dissipation, scalability, and cost minimization were some of the requirements imposed to the design. The system is composed of a current mode class‐D amplifier output stage and a voltage mode class‐D preamplification stage. The amplitude information of the radio frequency pulse was added through a customized step‐down DC‐DC converter with current amplitude feedback that connects to the current mode class‐D stage. Benchtop measurements and imaging experiments were carried out to analyze system performance. Direct control of B1 was possible and its load sensitivity was reduced to less than 10% variation from unloaded to full loaded condition. When using the amplifiers in an array configuration, isolation above 20 dB was achieved between neighboring coils by the amplifier decoupling method. High output current operation of the transmitter was proved on the benchtop through output power measurements and in a 1.5T scanner through flip angle quantification. Finally, single and multiple channel excitations with the new hardware were demonstrated by receiving signal with the body coil of the scanner. Magn Reson Med, 2013.

Interventional MRI‐guided brain biopsies using inductively coupled surface coils

The technical realization of inductively coupled surface coils for interventional MR‐guided procedures, and the application to brain biopsies in a 0.2 T magnet is described. The advantages compared to standard coils are discussed, and the results of 26 biopsies on eight different neuropathologic diagnoses from varying locations within the brain are presented. Initial experience shows that inductively coupled coils can offer an increased number of indications for interventional procedures in the brain, easier handling of sterility, and often a better access for the surgeon, compared to the use of standard MR head coils. Magn Reson Med 43:278–283, 2000.

Duplexer including a variable capacitance diode for an ultrasound imaging system

A duplexer for an ultrasound imaging-system capable of selectively coupling a transducer element to either a transmitter or a receiver, depending on whether the imaging system is in the transmit mode or receive mode, contains a variable capacitance diode which is connected in a direction opposite that of transmit pulses emitted by the transmitter between the transducer element and the receiver. In this manner, one obtains an improved duplexer that operates with virtually no power loss, without drive circuits, without leakage currents, and with low noise.

General design approach and practical realization of decoupling matrices for parallel transmission coils

In a coupled parallel transmit (pTx) array, the power delivered to a channel is partially distributed to other channels because of coupling. This power is dissipated in circulators resulting in a significant reduction in power efficiency. In this study, a technique for designing robust decoupling matrices interfaced between the RF amplifiers and the coils is proposed. The decoupling matrices ensure that most forward power is delivered to the load without loss of encoding capabilities of the pTx array.

Finite Element Computation of the Electromagnetic Fields Produced in the Body by Magnetic Resonance Imaging Surface Coils

The magnetic resonance imaging (MRI) Radiofrequency (RF) surface coil is a circular or rectangular conducting loop closed by a capacitor. In M.R. imaging, it plays the role of a magnifying glass, providing high resolution images with a high signal-to-noise ratio (SNR) on small regions of interest (ROI). Its main drawback is the non-uniformity of the magnetic field whichit produces, which leads to a loss of sensitivity, increasing rapidly with depth: this restricts the use of surface coils to the imaging of areas relatively close to the surface.